Role of Homogeneous Catalysis in Oligomerization of Olefins : Focus on Selected Examples Based on Group 4 to Group 10 Transition Metal Complexes

International audienceHomogeneous olefin oligomerization plays a pivotai role in the field of petrochemistry. Through catalysts, technology and process developments, market requirements in terms of productivity, selectivity and sustainability have been addressed. Over more than 50 years, an intensive research has been devoted to the design of new Group 4 to Group 10 transition metal complexes and to the study of their reactivity towards olefins leading to severa! breakthroughs of prime importance for academy as for industry. Since the early sixties, IFPEN contributed to bring innovative industrial solutions to different targets from gasoline production to alpha-olefin on purpose processes with over 100 production units built worldwide . Based on nickel, titanium, zirconium or chromium, the catalytic systems for such processes and their next generation are subject to a continuous research where the adaptation of the ligand architecture to the nature of the metal and its mode of activation, play a crucial role to control the reaction selectivity and the catalyst lifetime. lnteresting relationships between the complex structure and their reactivity have been drawn and will be discussed on selected examples

Silver(I) and copper(I) complexes with bis-NHC ligands : dinuclear complexes, cubanes and coordination polymers.

International audienceSilver(I) and copper(I) complexes containing neutral bis(N-heterocyclic carbene) (NHC) ligands and coordinated or non-coordinated chloride, bromide, iodide, or tetrafluoroborate anions, were synthesised. The nature of the anions impacts deeply the structural features of the complexes in the solid-state and neutral cubane-, neutral coordination polymer-, or dicationic bridged-type architectures have been characterised. The structures of (1,3-bis(3′-butylimidazol-2′-ylidene)benzene)disilver(I) dichloride (2a), bis(μ-1,3-bis(3′-butylimidazol-2′-ylidene)benzene-κ-C)tetra-μ3-bromotetrasilver(I) (2b), bis(1,3-bis(3′-butylimidazol-2′-ylidene)benzene)disilver(I) tetrafluoroborate (2d) in 2d·CH2Cl2, (1,3-bis(3′-butylimidazol-2′-ylidene)benzene)dicopper(I) dichloride (3a) and (1,3-bis(3′-butylimidazol-2′-ylidene)benzene)dicopper(I) dibromide (3b) were established by X-ray diffraction

Simplified and versatile access to low valent Ni complexes by metal-free reduction of Ni II precursors

International audienceThe reduction of [Ni(DME)Cl 2 ] with 2 equiv. of bis(trimethylsilyl)-1,4-tetramethyldihydropyrazine in presence of a ligand L and an excess of olefin cleanly leads to [Ni(L)(alkene) 2 ] complexes. When reduction is done in presence of 1,5-cyclooctadiene (COD), [Ni(COD) 2 ] is obtained. Such approach also allows access to the Ni I dimer [Ni(bis(dicyclohexylphosphino)propane)Cl] 2. Low valent, ligand stabilized (phosphines, N-heterocyclic carbenes = NHC) Ni(0) complexes are important species, being catalytically relevant in many transformations, such as cross coupling processes. 1 Among the known Ni(0) complexes, stable [Ni(L)(alkene) 2 ] or [Ni(L)(bis-alkene)] species (L=phosphines, NHC) are rare because of a lack of efficient, versatile synthetic route or availability of the appropriate precursors. 2 Indeed, [Ni(phosphine)(C 2 H 4) 2 ] have been reported by displacement of the poly-ene ligand from the extremely sensitive [Ni(CDT)] precursor (CDT = 1,5,9-cyclododecatriene), whereas other [Ni(L)(bis-alkene)] species have been prepared from air, light and heat sensitive [Ni(COD) 2 ]. 3-8 In turn, both Ni(0) complexes [Ni(CDT)] and [Ni(COD) 2 ] require the use of pyrophoric alkylaluminium derivatives to be synthesized. It is thus desirable to devise a rational, experimentally simpler, versatile method to access families of Ni(0) species. Recently, bis(trimethylsilyl)-1,4-tetramethyl-dihydropyrazine 1 has been shown to behave as a one or two-electron reducing agent for numerous transition metal complexes. 9,10,11 It was also used to reduce several late transition metal halide precursors to the corresponding metallic nanoparticles in the absence of stabilizing ligand. 12 In the case of Ni, nanoparticles active in carbon-carbon cross coupling process were obtained. We have hypothesized that if the in situ generated reduced Ni center could be efficiently trapped by ligands the aforementioned desirable Ni(0) complexes would be readily accessible. In the present work, we show that from the simple, commercially available, [Ni(DME)Cl 2 ] precursor (DME = dimethoxy-ethane), several Ni(0) complexes could be synthesized in one pot. We also report that with strongly donating bis-phosphine ligands, the reaction leads to Ni(I) complexes instead. Our work started with the synthesis of the emblematic and sensitive [Ni(COD) 2 ] complex. To date it is synthesized by reduction of Ni(acac) 2 with alkylaluminum compounds such as triethylaluminium or DIBAL-H in presence of COD and butadiene at low temperature, followed by low temperature crystallization and filtration to eliminate aluminium side products. 13 The room temperature reduction of [Ni(DME)Cl 2 ] with two equivalents of 1 was carried out in a first stage in THF with the presence of a large excess (20 equiv. of COD). Formation of black precipitate (likely Ni metal) pointed to a fast reduction compared to coordination by COD. The low temperature reduction (-78°C) was attempted to prevent Ni(0) precipitate formation, but the reaction does not proceed at this temperature within a day. The optimized reduction temperature was found to be at-20°C. In this case, we were able to synthesize [Ni(COD) 2 ] as crystalline material in 54% isolated yield, fully characterized by 1 H and 13 C NMR (Scheme 1). The moderate yield can be explained by the remaining formation of nickel black particles during the reduction, which can nevertheless be readily eliminated by filtration prior to crystallization of [Ni(COD)2] from cold toluene. Scheme 1 Synthesis of [Ni(COD) 2 ] by organic reduction of Ni(II) halide Beyond the synthesis of [Ni(COD) 2 ] itself, the reaction proved our hypothesis right, clearly indicating that monometallic Ni(0) can b

Outer-Sphere Reactivity Shift of Secondary Phosphine Oxide-Based Nickel Complexes: From Ethylene Hydrophosphinylation to Oligomerization

International audienceA new dimension for secondary phosphineoxide (SPOs) ligands is described in this article. Demonstratedon original p-allylic nickel structures, these self-assembledcomplexes trigger catalytic hydrophosphinylationreactions. Addition of a Lewis acid B(C6F5)3 switches the reactivitytowards migratory insertion and thus ethylene oligomerizationthrough an unprecedented outer-sphere interactionwith the coordinated SPO ligand. NMR experimentsand X-ray analyses allowed for the observation ofthe formation of zwitterionic active species as well as theirdegradation pathway

Synthesis and Characterization of Palladium(II) and Nickel(II) Alcoholate-Functionalized NHC Complexes and of Mixed Nickel(II)–Lithium(I) Complexes

The synthesis of Pd­(II) and Ni­(II) alcohol-functionalized N-heterocyclic carbene (NHC) complexes was explored to examine the possible influence of the functional arm attached to the NHC backbone on their structure and reactivity and, in the case of a Ni­(II) complex, on its catalytic properties in ethylene oligomerization. Starting from the alcohol-functionalized imidazolium salt [ImDiPP­(C₂OH)]Cl (<b>2</b>), the new functionalized NHC palladium­(II) complex [PdCl­(acac)­{ImDiPP­(C₂OH)-<i>C</i>_NHC}] (<b>3</b>) was synthesized and fully characterized. Two byproducts, [PdCl­{μ-ImDiPP­(C₂O)-<i>C</i>_NHC<i>,O</i>}]₂ (<b>4</b>) and <i>trans</i>-[PdCl₂{ImDiPP­(C₂OH)-<i>C</i>_NHC}₂] (<b>5</b>), formed during the synthesis of <b>3</b>, were also fully characterized. Acids promoted the transformation of <b>3</b> into the new C_NHC-bound complex [PdCl­(μ-Cl)­{ImDiPP­(C₂OH)-<i>C</i>_NHC}]₂ (<b>6</b>), unveiling the lability of the acac ligand and the resistance of the Pd–NHC bond to acids. Complex <b>6</b> reacted with a base to afford complex <b>4</b>, in which alkoxide coordination to Pd­(II) has occurred to generate a C_NHC,O chelate. The stability of <b>3</b> was also assessed under basic conditions, and the new complex [Pd­(acac)­{ImDiPP­(C₂O)-<i>C</i>_NHC<i>,O</i>}] (<b>7</b>) was characterized. The new nickel­(II) alcoholate-functionalized NHC complex [NiCl­{μ-ImDiPP­(C₂O)-<i>C</i>_NHC<i>,O</i>}]₂ (<b>8</b>) was synthesized by the reaction of the imidazolium salt <b>2</b> with <i>n</i>-BuLi and [NiCl₂(dme)]. The reaction of <b>8</b> with HCl regenerates the imidazolium and alcohol functions to give [ImDiPP­(C₂OH)]₂[NiCl₄] (<b>9</b>). The mixed-metal Ni­(II)–Li­(I) complexes [Ni₂{μ-ImDiPP­(C₂O)-<i>C</i>_NHC,μ<i>-O</i>}₄Li]­BF₄ (<b>10</b>), [Ni₂{μ-ImDiPP­(C₂O)-<i>C</i>_NHC,μ<i>-O</i>}₄Li]­Cl (<b>11</b>), and [Ni­{ImDiPP­(C₂O)-<i>C</i>_NHC,μ-<i>O</i>}₂LiBr] (<b>12</b>) were isolated and characterized. However, it was not possible to synthesize a Ni­(II) alcohol-functionalized NHC complex in high yield. Small amounts of the square-planar complex [NiCl₂{ImDiPP­(C₂OH)-<i>C</i>_NHC}₂] (<b>13</b>) could be isolated, and this complex was characterized by single-crystal X-ray diffraction. In <b>13</b>, only the C_NHC atom of the alcohol-functionalized NHC ligand is bound to the metal. The structures of the imidazolium salt <b>2</b>·2H₂O and of the complexes <b>3</b>, <b>4</b>, <b>4-polymorph</b>, <b>5</b>, <b>6</b>·CH₂Cl₂, and <b>8</b>–<b>13</b> were established by single-crystal X-ray diffraction

Bis(ether-functionalized NHC) Nickel(II) Complexes, <i>Trans</i> to <i>Cis</i> Isomerization Triggered by Water Coordination, and Catalytic Ethylene Oligomerization

The new nickel­(II) complexes containing NHC ligands N-substituted by a CH₂CH₂OR ether group (R = Me or Ph) [NiCl₂{ImMes­(C₂OMe)}₂] (<b>6</b>), [NiCl₂{Im<i>n-</i>Bu­(C₂OMe)}₂] (<b>7</b>), [NiBr₂{ImDiPP­(C₂OMe)}₂] (<b>8</b>), [NiBr₂{ImMes­(C₂OMe)}₂] (<b>9</b>), [NiBr₂{Im<i>n-</i>Bu­(C₂OMe)}₂] (<b>10</b>), NiBr₂{ImMes­(C₂OPh)}₂] (<b>18</b>), [NiI₂{ImDiPP­(C₂OMe)}₂] (<b>21</b>), [NiI₂{ImMes­(C₂OMe)}₂] (<b>22</b>), and [NiI₂{Im<i>n-</i>Bu­(C₂OMe)}₂] (<b>23</b>) were synthesized in good yields and fully characterized by NMR spectroscopy and X-ray diffraction analysis. The reaction conditions were optimized and further applied to thioether or nonfunctionalized NHC ligands, affording [NiBr₂{ImDiPP­(C₂SPh)}₂] (<b>19</b>) and [NiBr₂{ImDiPP­(<i>n-</i>Bu)}₂] (<b>20</b>), respectively. Equilibria involving <i>syn/anti</i> isomers were unveiled for complexes [NiCl₂{ImDiPP­(C₂OMe)}₂] (<b>5</b>), <b>6</b>–<b>10</b>, and <b>18</b>–<b>23</b>. Reactions of <b>6</b> and <b>20</b> with a halide abstractor afforded the dicationic aquo complexes <i>cis</i>-[Ni­{ImMes­(C₂OMe)}₂(H₂O)₂]­[PF₆]₂ (<b>27</b>) and <i>cis</i>-[Ni­{ImDiPP­(<i>n-</i>Bu)}₂(H₂O)₂]­[PF₆]₂ (<b>28</b>), in which a <i>cis</i> arrangement of the carbene ligands is evidenced, which contrasts with that in their precursors. These molecules represent rare examples of nickel aquo NHC complexes and of complexes with two <i>cis</i> monodentate NHC ligands. The new complexes reported in this work (15 crystal structures) displayed moderate activities as precatalysts for ethylene oligomerization and favored dimerization

Cationic Phenoxyimine Complexes of Yttrium: Synthesis, Characterization, and Living Polymerization of Isoprene

International audienceThe phenoxyimine cationic complex [L1Y(CH2SiMe2Ph)(THF)3][BArF] (L1 = (3-tBu)-(O)–C6H3–CH═N-(2,6-iPr-C6H3)) was prepared starting from the homoleptic [Y(CH2SiMe2Ph)3(THF)2] yttrium complex and the phenoxyimine ligand HL1 and the subsequent cationization by the anilinium borate salt. The resulting complex was characterized by different techniques such as elemental analysis, NMR (1H, 13C, and 89Y), and more specifically by the 1H-coupled 89Y INEPT. The reactivity of the cationic complex toward isoprene polymerization was evaluated in the presence of trialkylaluminum. This catalyst enables the living cis-1,4 polymerization of isoprene selectively. The influence and the role played by alkylaluminum are discussed based on the screening of a set of aluminum reagents. By means of a computational mechanistic investigation performed at the DFT level, a cationic complex that accounts for the cis/trans/3,4 selectivities experimentally observed is identified. Additionally, the in silico speciation of complexes resulting from the precatalytic mixture revealed the formation of stable Y/Al heterobimetallic complexes. Finally, for comparison purposes, the cationic amidinate yttrium complex [L2Y(CH2SiMe2Ph)(THF)3][B(C6F5)4] (L2 = PhC(N-2,6-iPr2C6H3)2) was synthesized and evaluated toward isoprene polymerization under similar conditions. This complex turned out to be active and selective toward the formation of 3,4-units

New boron-containing molybdenum imido alkylidene complexes for linear olefin homometathesis.

International audienceThe new molybdenum imido alkylidene complex Mo(N(2,6-iPr2C6H3))(CHCMe2Ph)(NC4H2Me2)(OB(Mes)2) (1;Mes = 2,4,6-MePh) containing both boroxide and pyrrolide ligands is reported. Its formation results from the reaction between bis(mesityl)borinic acid ((Mes)2BOH) and the bis-pyrrolide Schrock-type precursor Mo(N-2,6-iPr2C6H3)(CHCMe2Ph)(NC4H2Me2)2. The complex was fully characterized by 1H, 13C, 11B, and 95Mo NMR spectroscopy, X-ray diffraction, and elemental analysis. Complex 1 proved to be active for homometathesis reactions of 1- and 2-octene at 0.1 mol % loading. The synthesis of mixed pyrrolide boroxide imido molybdenum alkylidene complexes was extended to other borinic acids. The catalytic activity of these new complexes was evaluated in the homometathesis of linear olefins